The study of the organic light emitting device (OLED) was begun by A. Bernanose et al. at 1950s. In 1963, an organic electroluminescence device was fabricated by Pope et al with an anthracene single crystal. However, the first high efficient organic light-emitting device (OLED) fabricated by vacuum evaporation was an OLED developed by C. W. Tang et al in 1987, wherein aniline-TPD was used as a hole transport layer (HTL), and a complex of aluminum and 8-hydroxyquinoline-Alq3 was used as a light-emitting layer (EML). Its operating voltage was less than 10V, and its luminance was up to 1000 cd/m2. This breakthrough development made the field becoming a currently research hotspot. After entering 1990s, organic high molecular optical-electric functional materials entered a new development stage.
For the majority carriers are holes in organic EL device, it is necessary to improve the electron injection and keep the balance of electrons and holes in organic EL devices. In order to improve the electron injection and keep the balance of electrons and holes in organic EL devices, metals having a low work function for electron injection are required. These metals such as Ca, Mg or Mg:Ag alloy et al are, however, susceptible to atmospheric moisture and oxygen, which is a main reason for the device degradationi. Hungii et all, introduced an efficient bilayer cathode which consists of an ultra-thin LiF layer and an aluminum outlayer. However, the LiF have a relative high evaporation temperature, a thin optimal thickness (<1.0 nm) and also the improved electron injection ability requires the combinational reaction with the Alq3 and the rear cathode metal (Al).